1. Field of the Invention
The present invention relates generally to microwave phase and amplitude modulation structures and methods.
2. Description of the Related Art
Microwave transmission lines are conductive structures that form continuous paths from one point to another for transmission of electromagnetic energy. A transmission line conventionally includes two associated conductive members; a signal directing member that defines the signal path and a signal return member that completes a closed signal path. At low frequencies at which the signal wavelength is substantially longer than the transmission line structure, these members may take the form of two parallel wires and signal transmission can be analyzed in terms of member voltages and currents. At microwave frequencies (typically considered to be about 10.sup.9 -10.sup.12 Hz) the signal wavelength can be comparable to the size of the transmission line structure, and signal transmission is generally analyzed in terms of distributed circuit theory. At these frequencies, energy will rapidly radiate away from the transmission line unless one of the transmission members is configured to function as a containment or shield member.
Accordingly, microwave transmission lines typically include a signal member and a shield member. For example, a coaxial line has a usually cylindrical wire and a hollow cylindrical shield that is coaxially arranged with the wire. In this transmission structure, the electromagnetic field is completely enclosed between the signal and shield members.
As a second example, a stripline transmission line has a rectangular signal member positioned between a pair of parallel, flat shield members that are typically referred to as "ground planes". In this transmission structure, the electromagnetic fields are no longer completely enclosed but the shield members preferably extend sufficiently in the line's transverse direction to contain the majority of the electromagnetic fringe field.
The dimensions of a transmission line's signal and shield members and the signal line's termination impedance determine the line's impedance at any other point. With a specified termination, the impedance along the transmission line becomes a function of the distance from that termination. For example, if the signal member of a transmission line is terminated in an open circuit (high impedance), the line impedance will be a low impedance at a point .lambda./4 from the open circuit and will again be a high impedance at a point .lambda./2 from the open circuit (in which .lambda. is the signal wavelength).
Microwave signal energy can be effectively coupled through an aperture that is formed in the shield member of a transmission line. For example, Pozar has analyzed two antenna structures that are based on the use of shield member apertures (Pozar, David M., "A Reciprocity Method of Analysis for Printed Slot and Slot-Coupled Microstrip Antennas", IEEE Transactions on Antennas and Propagation, Vol. AP-34, No. 12, December, 1986, pp. 1439-1446). In the first structure, an electromagnetic signal is radiated directly from a microstrip signal member through an aperture in the shield member. This antenna structure is called a "microstripline-fed printed slot". The second structure is an "aperture-coupled patch antenna". In this structure, the aperture is positioned between the signal member and a radiating patch member. The patch is generally a square or rectangular conductive sheet that is printed on a dielectric substrate.
Transmission line shield apertures can be used to effectively reduce the size of coupled signals in multilayer, microwave transmission structures. For example, Hersovici and Pozar (Herscovici, Naftali, I. and Pozar, D. M., "Analysis and Design of Multilayer Printed Antennas", IEEE Transactions on Antennas and Propagation, Vol. 41, No. 10, October 1993, pp. 1371-1378) provide an illustration (FIG. 1) that shows a multilayer stripline structure in which 1) a first shield aperture couples radiation from an entry stripline to a primary stripline feed network, 2) a set of shield apertures couples energy from the primary feed network to a secondary stripline feed network, and 3) a second set of shield apertures couples energy from the secondary feed network to the patches of a patch antenna.
Although the control of microwave energy flow in compact microwave circuit designs is facilitated by apertures in shield members, phase modulation and amplitude modulation circuits are typically realized with configurations that require relatively greater space. For example, a phase modulator may have two phases which are realized by switching between signal members that have different path lengths. In another configuration, a signal member of a fixed path length is terminated with a variable reactance device.